The invention relates to an electric machine having grooves and windings of electrical conductors, wherein the windings are divided into strands, wherein at least one strand has a respective number s of coils connected in series, which are each are arranged in the grooves, wherein the coils each comprise sub-conductors connected in parallel, wherein the sub-conductors of a coil are in each case arranged in a plurality of bundles, wherein a bundle of a coil is in each case connected in series to a bundle of a coil connected in series, wherein each bundle of the at least one strand is in each case arranged in a groove at a respective bundle position in relation to a deepest possible bundle position in the groove, wherein the respective bundle positions of all bundles in all coils of the at least one strand in each case define a respective number b of possible bundle positions of the at least one strand.
In the case of electrical conductors through which alternating current flows, the current density inside a conductor is lower than it is on the surface. This effect, known as current displacement or skin effect, is also observed for an electric machine comprising windings and turns arranged in grooves. In particular, in the case of high feed frequencies, these current displacement phenomena between the sub-conductors (so-called first-order current displacement) result in massive supplementary losses.
To date, the following measures have been used to reduce these supplementary losses, in particular in the case of coil windings. For example, a bundle interchange is performed with which the parallel sub-conductors are divided into two bundles, wherein the upper and the lower bundle are interchanged approximately in the middle of the coil group. This measure is very similar to bar transposition with bar windings, which is known as a Roebel bar. Alternatively, it is also possible to use a sub-conductor transposition for which the sub-conductors are transposed 180° in a short distance at a specific position either inside a coil or between two coils of a coil group.
A device of this kind is, for example, known from DE 1 041 148 A. Here, conductive bars are embodied as Roebel bars in that with a conductive bar, the sub-conductors are inclined into two planes and offset from one conductor plane into the other conductor plane.
The invention is based on the object of reducing the electrical resistance for an electric machine that has windings arranged in grooves and can be operated with alternating current.